At least where I live, I think it comes down to the whole culture surrounding alcohol and fast-food, which is ingrained since childhood or highschool.
In my year, many began to gain weight around the ages of 16 and 17 which is also around the time people started to get their drivers license and the time when alcohol and marijuana use became most pervasive. My year was small so everyone knew everyone and everyone knew nearly everything that was going on so I can pretty confidently say almost everyone was drinking, about 40% were regularly drinking, over half of those were binge-drinkers and about 15-20% were regular uses of pot, and most people who smoked pot were the boys. People kept pot in their lockers and would mix vodka with cordial in a stainless-steal drinkbottle and take it to class. Teachers became aware and lockers started to be inspected weekly and drinkbottles were only allowed in clear plastic. That didn't really stop the problem though because parents would supply their underage 16 and 17 year old children with alcohol to drink casually in a home environment or supply them before they went off to a party (note, this is normal across Australia).
Concerning driving, it is a necessity here to get your license sooner rather than later given it is semi-rural and public transport is shit. However, most who had their license also completely stopped walking places or riding their bikes. People also start going on maccas or fish and chip runs after school or after parties, loading up on calories they wouldn't normally have and don't otherwise need.
Since graduating, I keep in contact with almost everyone on Facebook. Most in my year didn't start off overweight and not everyone did become overweight. Majority gained weight but I would say only about 20-30% are overweight. Some people who stayed skinny for that period have started gaining weight whereas others are making a conscious effort to lose and have been quite successful. Fortunately most try to make healthy food choices and many are part of a local sports team, go to the gym or try to prepare for the occasional fun run. I'm doing the colour-run this year with 4 people from highschool and I expect I will be running into (and hopefully overtaking), other people I graduated with there too.
tl;dr _ Alcohol, drivers license and poor food choices make people fat in their later years of high-school or after they graduate.
But i just find it so inspiring if i see a pretty girl with a great sense of fashion. 
I used to wonder if they knew something that I didn't, you know? The fact that it's cultural rather than biological puts my mind at ease 
